Reciprocating air pumps have been adopted as the most common tools for the inflation of inflatable articles in which the required inner pressures are within a comparatively higher range. On the other hand, in the inflation operation for inflatable articles of lower inner pressures, the most popular tools have been multi-stage centrifugal compressors which, instead of compressing the air to a higher pressure at a speed quite low as for reciprocating air pumps, can compress the air in a more speedy manner as compared with the reciprocating types.
A conventional multi-stage centrifugal compressor or air pump as shown in FIGS. 1 through 3 generally comprises: an intake conduit 1 for introducing the air into the air pump; a motor chamber 3 received therein with a motor 31 whose shaft 32 provides rotation power to the air pump; a multi-stage centrifugal compressing section 2 composed of several compressing units 5 which are connected in series and are driven by the motor shaft 32 for compressing the air by centrifugal force; a discharge conduit 4 for introducing the compressed air into the interior of an article to be inflated. Each compressing unit 5 (see FIGS. 2 and 3 wherein two consecutive compressing units 5 are shown) comprises a rotor 51 which is fixed to the motor shaft 32 and can rotate together with the shaft 32, and a stator 52 which is fixed to the housing of the air pump. The stator 52 comprises a disk-shaped spacing plate 56 having a central hole 58 provided at the center thereof, and an annular portion 57 integrally formed with and perpendicular to the periphery of the disk-shaped spacing plate 56. Each rotor 51 is received within the cavity formed by the spacing plate 56 and the annular portion 57 of a stator 52. The rotor 51 comprises a generally disk-shaped base plate 53, a plurality of blades 54 integrally formed with and supported by the base plate 53, and a hollow hub 55 protruding axially from the central portion of the base plate 53 whereby the rotor 51 may be fixed to the motor shaft 32 by fitting the hub 55 onto the shaft 32. FIG. 3 illustrates how the parts shown in FIG. 2 are assembled onto the motor shaft 32 to form two compressing units 5, and how the air is forced to flow through each compressing unit 5 along the directions of the arrows shown in FIG. 3 and compressed. The air to be compressed enters each compressing unit 5 from the right side, and is forced to move radially outwards by the centrifugal force produced due to the rotation of the blades 54 of the rotor 41. Then, when the air reaches the annular portion 57 of the stator 52, it is forced to overpass the outer periphery of the base plate 53 and reach the back end of the latter through the gap between the outer periphery of the base plate 53 and the inner periphery of the annular portion 57. Further, the air is forced to enter the adjacent compressing unit 5 through the central opening 58 of the spacing plate 56, thereby completing one stage of the air compression operation. Similar processes are repeated in the compressing units 5 connected in series so as to finally complete the whole multi-stage compression of the air. However, due to the fact that the air to be compressed must pass through a very narrow, long and winding passage during the compression operation, this type of multi-stage centrifugal compressor or air pump still suffers from the drawback of being incapable of inflating at a satisfactory speed, especially for the inflation of an inflatable article having a comparatively large volume when inflated.